Because they need a bath? He has them one weekend day a week, typically they don't have a bath on that day. Several times he's brought them home an hour and half before they're due, because they "need a bath" (have been to the park or similar). They have a bath the day before and the day after and are both under 5 so, not filthy regardless of how lucky their clothes got.
I don't see the need to cut this one day of child free time short and find it somewhat annoying!